Transcript Light scattering by feldspar particles

Light Scattering by Feldspar Particles:
Modeling Laboratory Measurements
Evgenij Zubko1,2, Karri Muinonen1,3, Olga Muñoz4, Timo Nousiainen1,
Yuriy Shkuratov2, Wenbo Sun5, and Gorden Videen6,7
Department of Physics, University of Helsinki, Finland
2 Institute of Astronomy, Kharkov National University, Ukraine
3 Finnish Geodetic Institute, Finland
4 Instituto de Astrofísica de Andalucía, CSIC, Spain
5 Science Systems and Applications, Inc., USA
6 US Army Research Laboratory, USA
7 Space Science Institute, USA
1
Laboratory measurements of single-scattering feldspar particles
appear to be a huge challenge for modeling
Data adapted from Volten et al. 2001: JGR 106, pp. 17375–17401
Results of fitting feldspar at 0.442 m
Data adapted from Dubovik et al. 2006: JGR 111, D11208
Results of fitting feldspar at 0.633 m
Data adapted from Dubovik et al. 2006: JGR 111, D11208
Little difficulties in modeling
Unfortunately, the parameters were different for the different
fits, and
“Simultaneous inversions of scattering matrices measured at
two wavelengths (0.442 m and 0.633 m) were not
successful in that a reasonably good fit was not achieved.
The root-mean-square (over all elements) fit for a single
wavelength was about 7–10%, while for two wavelengths the
root-mean-square fit did not drop below 20%.”
from Dubovik et al. 2006: JGR 111, D11208
Little difficulties in modeling
Besides, the best fits are obtained with a mixture of highly
oblate and prolate spheroids. However, the feldspar particles
look highly irregular with aspect ratio being somewhat about 1.
from Dubovik et al. 2006: JGR 111, D11208
Modeling feldspar with agglomerated debris particles
Method: Discrete Dipole Approximation (DDA)
Concept: Modeling target with set of small sub-volumes
Advantage: Arbitrary shape and internal structure
Modeling feldspar with agglomerated debris particles
More details in, e.g., Zubko et al. 2009: JQSRT 110, pp. 1741–1749
Modeling feldspar with agglomerated debris particles
Features of agglomerated debris
particles:
(1) Highly irregular
(2) Equi-dimensional
(3) Fluffy (packing density =0.236)
Laboratory measurements of single-scattering feldspar particles
SEM image of feldspar
Refractive index m is estimated
to be in range
m = 1.5–1.6 + 0.001–0.00001i
Data adapted from Volten et al., 2001
Size distribution is retrieved with
the laser diffraction method
Modeling laboratory measurements of feldspar
We consider the range of particle radii
r from 0.21 m through 2.25 m
at =0.442 m: x=3–32
at =0.633 m: x=2.1–22.3
SEM image of feldspar
Size distribution:
r–2.9
Size parameter: x = 2r/
Refractive index:
m = 1.5 + 0i
For each size parameter x, we consider a minimum of 500
samples of agglomerated debris particles in random orientations.
This makes our analysis being statistically reliable!
Modeling laboratory measurements of feldspar at =0.442 m
Modeling laboratory measurements of feldspar at =0.633 m
Summary
We model light-scattering response measured in feldspar
particles at two wavelengths 0.442 and 0.633 m. We utilize
model of agglomerated debris particles and compute light
scattering with the discrete dipole approximation (DDA).
Measurements can be satisfactorily reproduced under realistic
assumptions on size distribution and refractive index of
feldspar particles. Unlike spheroidal model, agglomerated
debris particles can fit measurements at both wavelengths.